There are a number of different ways by which integrated circuit ("IC") devices may be bonded to a printed circuit board using localized techniques, such as thermo-compression bonders, so called hot bars, or lasers. The ultimate objective of any bonding approach is to insure that a reliable physical and electrical connection is formed between the leads, which are connected to or formed with the device, and the contact pads, which are integrally formed with the board.
Certain difficulties are encountered using devices such as thermo-compression bonders or hot bars when the board involved in the bonding process includes a reference plane within the substrate. More specifically, when one attempts to perform localized, surface mount bonding using such devices the reference plane within the substrate, which has a very large mass relative to the contact pads, operates as a thermal sink tending to draw heat away from the lead/pad combinations to be bonded. This is especially so when the reference plane is close to the surface, or when the pad is connected to the plane by means of a via. Consequently, the teaching in the bonding art suggests that the presence of the reference plane will tend to interfere with the use of localized bonding techniques. To address this problem, the use of thermo-compression bonders and hot bars typically require the raising of the temperature of the substrate and the reference plane, by pre-heating for example, in order to successfully accomplish bonding. This, of course, makes for a more involved bonding process.
Laser bonding has been found to produce more reliable lead/pad joint formation and has tended to provide higher product yields relative to other approaches. One such laser bonding approach is single point bonding, involving the use of a pulsed beam. A pulsed beam is one which either actually or effectively turns on and off during operation. In single point bonding the laser is positioned so that the beam will be directed at a single lead/pad combination and irradiated to effect reflow of adjoining metals. After a certain interval the transmission of the beam is stopped, and the board and laser are then repositioned relative to one another so that the next lead/pad combination can be irradiated. After irradiation, the melted solder of the first lead/pad combination solidifies to form the bond.
One drawback to the single point bonding technique is that a very small beam spot size is required, which is technically difficult to achieve and maintain. Secondly, the beam that is emitted by the laser must be precisely positioned relative to the lead/pad combination so that the beam is directed at only one lead/pad combination at a time. Furthermore, the beam alignment must be precise so that the spot irradiates only the lead/pad combination, and not the section of the board that lies in between adjoining lead/pad combinations. This is to avoid burning the board with the high energy density beam characteristic of finely focused laser output. Finally, it is also technically difficult to achieve pulse to pulse stability in the single point bonding approach.
A different laser bonding approach is single pass, continuous wave bonding, hereinafter referred to simply as "CW" scanning. Through this technique the laser beam is continuously emitted as the beam scans around the lead/pad combinations of the component positioned on the board. Such beam scanning can be accomplished by the movement of the board relative to a stationary laser beam, or the movement of the beam relative to a stationary board.
Unlike the single point bonding approach, in CW scanning the laser does not effectively turn on and off. Instead, the laser beam remains on throughout the time that it is directed at the leads of a given component. Consequently, during the operation of the CW scanning system, portions of the board between adjoining lead/pad combinations will necessarily be irradiated by the laser.
In processing boards using CW scanning, the irradiation of the lead/pad combinations and, consequently, the board requires the consideration of a number of important factors. Firstly, during CW scanning the laser must irradiate the lead/pad combinations for a long enough period of time to effect reflow of lead/pad solder, which ultimately solidifies to form a joint. Although it may be desirable to prolong the duration of the irradiation of the lead/pad combination or use a high powered beam, in order to insure the necessary reflow, such an approach may cause the board to be thermally damaged during the process. Additionally, any delay of the movement of the beam, beyond that which is necessary to cause reflow, has an impact on the efficiency of the board processing operation.
Efficient and successful CW scanning therefore requires the understanding of the effect of the various factors that influence the success of the process. Accordingly, it is desirable to have a delineation of the CW scanning process factors, and a method of using those factors for the CW scanning of printed circuit boards.